1. Field of the Invention
The present invention relates to hard disk drives (HDDs). More particularly, the present invention relates to a technique for detecting and eliminating an electrical potential difference between a slider body and a disk surface of an HDD, such as a contact potential caused by material differences between the slider and the disk or a potential generated by tribocharging.
2. Description of the Related Art
FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a dual-stage servo system for positioning a slider assembly 101 over a selected concentric data information track on a magnetic disk 102 for writing data to and/or reading data from the selected track. The dual-stage servo system of HDD 100 includes a primary actuator 104, such as a rotary voice-coil motor (VCM), for coarse positioning an actuator arm 105 and a read/write head suspension 106, and a secondary actuator (not shown in FIG. 1), such as a microactuator or micropositioner, for fine positioning slider assembly 101 over a selected track. A microactuator, as used herein, is a small actuator that is placed between a suspension and a slider and moves the slider relative to the suspension. Slider assembly 101 includes a read/write head (not shown in FIG. 1) having a read element, such as a Giant Magnetoresistive (GMR) element, and a write element that respectively read data from and write data to a selected data track. While HDD 100 is shown as having only a single magnetic disk 102, HDDs typically have a plurality of stacked, commonly rotated, rigid magnetic disks and a corresponding number of actuator arms, read/write head suspensions, secondary actuators and slider assemblies.
As slider-to-disk spacing becomes smaller than 10 nm, electrostatic and intermolecular forces between a slider and a disk become increasingly significant. Even when a slider body and a disk are both grounded, a potential difference can exist between the slider body and the disk that can generate an electrostatic force greater than the van der Waals force. One source of the potential difference is tribocharging, or frictional electrification of non-conducting materials on the slider body. (See, for example, J. D. Kiely et al., “Tribocharging of the magnetic hard disk drive head-disk interface,” Journal of Applied Physics, Vol. 91, No. 7, pp. 4631–4636, Apr. 1, 2002.) Another source is the contact potential between the conducting materials of the slider body and the disk.
FIG. 2 depicts an electrostatic field that can exist between a slider body 201 and a hard disk 202 of a hard disk drive. A suspension supporting slider body 201 is not shown. As disk 202 rotates, disk 202 moves from right to left with respect to slider body 201, as indicated by arrow 203. Enlargement 204 of the slider-disk interface shows lines representing an electric field 205 that is formed from a potential difference between slider body 201 and disk 202. Electric field 205 exists between slider body 201 and disk 202 all along the length of slider body 202, but is only indicated in enlargement 204 because the intensity of electric field 205 is greatest at the trailing edge of slider body 201. As mentioned, one source for the potential difference is the contact potential that originates from the conducting portions of the slider body and disk having different work functions and from tribocharging of the non-conducting portions. Another source for the potential difference is tribocharging associated with the spindle motor bearing, which can shift the disk potential significantly from ground potential.
Electric field 205 between slider body 201 and disk 202 generates an electrostatic force that acts the on the surfaces of slider body 201 and disk 202 within the slider-disk interface. The electrostatic force associated with electric field 205 depends on the applied voltage and the other physical parameters of the slider-disk interface. As the potential difference between the slider body and the disk increases, the slider flying height is reduced from the design flying height of the slider.
For example, FIG. 3 shows the flying height (FH) for a high-pitch air bearing design and a low-pitch air bearing design as a function slider-disk potential difference. Both air bearing designs have a design fly height 9 nm above a disk. The high-pitch slider has a dynamic pitch angle of 180 μrad. The low-pitch slider has a dynamic pitch angle of 70 μrad at a flying height of 9 nm. Curve 301 represents the flying height for the high-pitch slider as a function of the slider-disk potential difference. Curve 302 is the flying height for a low-pitch slider as a function of the slider-disk potential difference. The low-pitch slider is more sensitive to an applied DC electric field than a high-pitch slider because a low pitch design has on average a smaller separation distance than a high-pitch design when the electrostatic force is integrated over the slider. The flying height is reduced by about 0.5 nm for a potential difference of about 0.5 V for both the high-pitch and low-pitch air bearing designs. The value of the flying height at 2.5 V for the low-pitch slider is extrapolated to 0 nm because contact occurs at 2.5 V.
In current hard drives, the disks are lubricated using perfluoropolyether-type lubricants having long polar chains, such as Fomblin Z-DOL. The polarity of the lubricant in the presence of an electrostatic field at the head-disk interface can cause lubricant to be removed from the disk surface and possibly be depleted from the disk surface. The removed lubricant is usually picked up by the slider and can lead to drive failure. Consequently, as lubricant is removed, the lubricant is attracted to slider body 201 by the polarity of the lubricant and the electrostatic force and becomes concentrated in areas on slider body 201 that have a high electric field, such as the trailing edge of the slider body. Once concentrated, the lubricant can form droplets that can lead to read/write errors when the droplets of lubricant enter the head-disk interface. Electric field 205 can also attract particles onto slider body 201 that can lead to scratches on both slider body 201 and the surface of disk 202. Yet another potential problem that is caused by the electrostatic force is an increased vibrational coupling between slider body 201 and disk 202, leading to larger modulations of the slider-disk spacing.
A number of researchers have found over the past years that slider-to-disk spacing can be controlled by adjusting a DC bias applied to the slider-disk interface. See, for example, U.S. Pat. Nos. 6,005,736 to E. T. Schreck; U.S. Pat. No. 6,529,342 to Z. Feng et al.; and U.S. Pat. No. 6,366,416 B1 to D. W. Meyer et al.
Accordingly, what is needed is a technique for determining the optimum bias voltage that should be applied between a slider body and a disk for eliminating an electrical potential difference that exists between the slider body and the disk, such as a contact potential.